$post_data = http_build_query($req, '', '&');

So how would one take the PHP code:

$post_data = http_build_query($req, '', '&');

And make it relevant in xojo… $reg is being replaced by a dictionary variable req.
is there any kind of http_build_query function in xojo ?

HTTPSocket has a function to set form post data as dictionary. It will encode it internally.

can i use it to store in a string that i then use for optionPostFields with MBSCurls ?

Someone had a vb.net example written up

Dim postData As String = "method=cancelallorders&nonce=" & random

That makes more sense… i think i can just structure it myself.

you can encode it yourself. With EncodeURLParameter and = and & as divider.

Or you use FormAdd methods in plugin to build a form yourself.

Depends of course if you want to have simple post parameters or form parameters with multiple sections.